Cesium light source



Dec. 30, 1969 DQH. PoLLocK 3,487,252

CESIUM LIGHT-SOURCE Filed Jan. 22, 1968 2 Sheets-Sheet 1 Dec. 30, 1969"n.l-l; PoLLoCK CIESIUM LIGHT 'SOURCE n. u. h.

Filed Jan. 22, 1968 QQN E W M .w m S m H NS. H w W h s 2 ,W SM y SVM@ fhm NQ United States Patent O U.S. Cl. 313-180 11 Claims ABSTRACT OF THEDISCLOSURE A vapor electric discharge tube having an electrode made of apure refractory metal and having a temperature controlled reservoir ofthe vaporizable metal. Heat insulation is provided between the reservoirand operating region of the device so that the temperature of thereservoir may be independent of the temperature of the operating region.

CROSS REFERENCES TO RELATED APPLICATIONS This is a continuation-in-partof my copending application Ser. No. 501,613, filed Oct. 22, 1965, nowabandoned.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to an electric discharge tube which serves as a gas plasma arcdischarge device.

Description of the prior art Vapor discharge lamps utilizing avaporizable metal typically utilize an alkali metal to provide a gasplasma for the arc discharge. Examples of such lamps in the prior artachieve the power output by ordinary methods such as increasing thetemperature of the cathode until suicient current is produced to providean arc of the desired radiated output. To describe this in another waythe vapor pressure within the unit may be varied with the correspondingchange in the temperature to produce the desired radiated output. Forhigh radiated outputs, typically the temperatures of the electrodes areso high that rapid degradation of the cathode material occurs withreduced light output and a result in short life of the device.

'I he present invention is directed to a vaporizable metal dischargedevice which can operate at high outputs with relatively low operatingtemperatures and thus high efficiency.

SUMMARY OF THE INVENTION The Ipresent invention provides a novelelectric discharge device construction which permits adjustment of thevapor pressure and has a pure refractory metal electrode. The vaporpressure is adjustable `by including in the device a temperaturecontrolled reservoir of the vaporizable metal which metal is utilized inthe discharge arc. The reservoir is separated from the main arc regionby a mechanical heat choke section so that the reservoir is always atthe coolest temperature of the device. A heating coil surrounds thereservoir and by adjusting the current in the coil the temperature ofthe reservoir can be varied as desired thus controlling the vaporpressure. One of the electrodes in the present invention comprises apure refractory metal such as tungsten, for example.

3,487,252 Patented Dec. 30, 1969 ICC BRIEF DESCRIPTION OF THE DRAWINGFIGURE 1 is a cross sectional view of device ernbodying this invention;

FIGURE 2 is a sectional view taken along line 2-2 of FIGURE 1;

FIGURE 3 is a schematic View of the cuit and the temperature sensingmeans of the invention; and

FIGURE 4 is a graphical representation of current density as a functionof temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although a number of vaporizablemetals such as, for example, rubidium, mercury, lithium, and franciumare capable of utilization in a vapor discharge arc device the metalcesium is readily available and has particularly desirablecharacteristics for this use and, accordingly, the invention herein,although not so limited, will be described with respect to cesium. n

The construction of a successful cesium vapor-electric discharge devicehas long been a desired goal since cesium has a number of advantages notheld `by mercury or other vaporizable metals. For example, cesiumpossesses one of the lowest ionizable potentials of the available metalsand, therefore, a cesium vapor discharge device operates at a lower arcdrop for a particular current than devices using other vaporizablemetals. It has been discovered in the invention herein that cesium in adischarge device coats the electrodes during operation of the arc andprovides extremely efcient electron emission from the coated electrodes.This cesiation of the electrodes provides higher eiciency of operationand, thus, lower operating temperatures which reduces tube blackeningand increases the life of arc-discharge devices.

The employment of cesium as an ionizable medium in an electric dischargedevice presents many problems. Minimum operating voltage for eflicientarc operation is dependent upon and inversely proportional to the vaporpressure of the vaporizable metal used in the discharge arc. Therefore,to achieve low operation voltage and high eiciency, the vapor pressuremust be maintained at a relatively high value. Further, high vaporpressures allow the arc-discharge device to operate at a lower currentfor a particular radiated output. This results from the greatconcentration of cesium ions in the arc region at high vapor pressuresso that less current is required to produce the desired radiated output.That is, a higher arc voltage can be maintained and, thus, a lowercurrent so that less heating loss through the electrodes results. Ifcurrent is increased in the arc-discharge device, a point will bereached when more cesium atoms leave the electrodes than arrive so thatthe electrodes may no longer have even a monoatomic layer` of cesiumthereon. Thus, the current decreases and the electrodes cool. This maycause the operation point of the device t0 oscillate. To avoid suchproblems, it is necessary to increase vapor pressure when current isincreased to maintain proper cesiation of the electrodes and to have acathode construction which is conducive to good cesiation.

Referring to the drawing, a preferred embodiment of the invention isshown. The device shown is a metal vapor discharge-arc device embodyingthe invention. The device comprises generally two sections or portions,a tubular housing A and a tubular control section B.

Housing A comprises a tube 10 which is closed at the ends by electrodeholders, namely anode mount 11 and cathode mount 12.

Tube 10 is cylindrical and in the preferred embodiment of the inventionis made of fused alumina. Although other tube materials can be utilized,fused alumina was chosen heating coil cirbecause of its high capabilityto withstand heat which capability is higher than that of quartz andbecause of its capability of being easily brazed to metals. Thus, fusedalumina is an economical and practical tube material for use in anarc-discharge device that operates at high temperatures. Furthermore,fused alumina is Substantially transparent in the infrared andnear-infrared regions.

Provided at the ends of tube are mounts 11 and 12 which are made ofnickel or other high temperature metal. The mounts 11 and 12 aregenerally cylindrical and have recessed sections 15a and 15b which haveinside diameters substantially equal to the outside diameter of tube 10.The recesses 15a and 15b are cooperatively press fitted to tube 10 atthe ends thereof and are brazed into sealing engagement therewith. Thebraze material may be copper or any other high temperature brazematerial such as, for example, an active alloy braze, that is, any brazethat can satisfactorily withstand temperatures of 1000 C. or over.

Mounts 15a and 15b dened therein circular openings 16a and 1611 whichare extended by circular flanges 17a and 17b. Cathode holder 20 is pressfitted through flange 17b. The cathode holder 20 has a larger diameterrear portion 21 which defines a shoulder 22. Shoulder 22 abuts againstthe end of flange 17b and the assembly is brazed together for sealing.Cathode holder in its forward portion contains a cylindrical recess 24which is sized to receive the generally cylindrical cathode 25 in apress t. Cathode 25 is welded to holder 20. A reduced diameter portion26 is provided in holder 20 for ease of welding; that is, the thinnerportion of metal becomes somewhat tiuid during the welding process andflows into contact with the cathode 25 thus making a secure and reliableweld. It is not necessary to have portion 26, but it provides for morereliable welding and, therefore, is convenient for ease of manufacture.The cathode 25 is made of tungsten which is substantially pure forreasons which will be discussed hereinafter.

Cathode holder 20 is made of stainless steel, but can be fabricated ofany suitable high temperature metal.

Control section B has a recess 31 and a reduced diameter portion 32 isprovided to hold anode 33 in the same manner as is cathode 25 asdescribed hereinabove. Control section B is brazed to mount 11 at iiange17a in the same manner, described hereinabove, as is holder 20 to ange17b. Extending coaxially through control section B is cylindrical bore50 which is open at end 51 of section B. A cylindrical bore 56 isprovided near plane 55 and oriented transversely to the axis of bore 50.Bore 56 is located within housing section A. Thus, as is easily observedin the drawing, bore 56 provides communication between housing section Aand bore 50.

Section B is cylindrical and made of stainless steel or other metalscapable of withstanding the temperatures of 1000 C. or over which areencountered in operation of the device. A necked down portion 60 isprovided in section b to create a heat choke. That is, since the heatflux created by the arc is constant, the reduced cross-sectional area ofthe necked down portion 60 will cause the temperature change per unitlength to be greater for the necked down portion 60 and, thus, asignificantly greater temperature differential will exist across portion60 than would exist if no reduced diameter section was included. Becauseof the large differential of temperature across portion r60, thetemperature of portion 61 of the control section B will be substantiallycooler than section 62 and, thus, substantially cooler than temperaturesin the vicinity of the arc in housing section A. All cesium thatcondenses in the device Will condensefat the coolest point and, thus,will condense the bore in portion 61. Thus, portion 61 provides insubstance a chamber or reservoir in which all condensed cesium of thedevice is contained. v

Disposed about portion 61 is an electrical heating coil 70 shownschematically in FIGURE 1. The heating coil 7() is connected to a powersource 71 and the euri-ent through coil70 is adjusted or controlled bymeans of rheostat 72.

Any heating means, as for example a gas burner, may be utilized;however, a heating coil is relatively inexpensive and easy to controland, thus, was chosen for the preferred embodiment of the invention.

Connected to portion 61 is a cap section 75 which is welded at flange76. The cap section 75 is used for initial charging and sealing of thedevice and can be of any convenient shape or size. A thermocouple isinstalled in the wall of cap section 75 for measuring the temperaturewithin section 75. This measured temperature is close to the temperaturewithin portion 61 because of proximity of the areas. Necked downportions 81 and 82 are provided in the electrodes for providingelectrode heat isolators, that is, to keep the posts of the device whichare not close to the arc relatively cool. The shapes of the electrodesas shown in FIGURE l ywere designed for eiiicient arcv operation.

Nickel wires and 91 are welded to the electrode holders and are adaptedfor connection to a power source for operating the arc.

A predetermined quantity of cesium or other vaporizable alkali metalsuch as any of the metals from Group I of the Periodic Table or materialis sealed with the device.

It is a very important feature of this invention that the cathode bemade of pure refractory metal. It is not necessary to have the cathodemade of a single refractory metal, it may be made of a combination ofsuch metals. However, it is to be noted expressly that if desired thecathode may be made of a single refractory metal. In an embodiment ofthe invention which has been lab tested, the electrode was made oftungsten and was approximately 99.99% pure elemental tungsten. Othermetals have been utilized such as, for example, the refractory metals ofmolybdenum, rhenium, zirconium, niobium and tantalum. Tests and otherdata indicate that a cathode made of any combination of this group ofmetals, which include tungsten, will be satisfactory for the inventionprovided that there is no greater than .1% other matter in thecomposition of the cathode. Accordingly, it is believed that for thepurposes of this invention that the cathode is substantially purerefractory metal if its composition is 99.9% or more refractory metals.

For a clearer understanding, an explanation of the operation of theinvention is desirable. In operation, wires 90 and 91 are connected to apower source. The voltage of the power source is raised until an arc isstruck between electrodes 25 and 33. The arc generates heat whichionizes the cesium in the system, which makes the arc self-sustaining,this is, the voltage across the arc equalizes at a level sufficient tosustain the arc which level is substantially lower than the breakdownvoltage required to strike the arc. Cesium will condense at the coolestpoint of the system which is, as discussed hereinabove, in reservoir orchamber 61. Thus, the temperature of chamber 61 will determine the vaporpressure and the operating point of the device. To change this operatingpoint, it is merely necessary to changey the temperature of portion 61by varying the current through heating coil 70 by rheostat 72. The vaporpressure can be measured by interrelating it to the temperature ofportion 61 which can be sensed by an temperature sensor such as, forexample, thermocouple 80.

An experimental model made in accordance with this invention has anoverall length of approximately five inches, an outside diameter ofapproximately 0.5 inch and is comptued to be capable of operating atinput powers of 1000 watts or more. Spectrometer analyses of the modelshow that the device can produce an output in which 80% or more of theradiated energy occurs at wavelengths of greater than one micron; thus,the device provides an extremely eiiicient infrared source.

Referring to FIGURE 4 of the drawing, the graph therein shown hasplotted in the abscissa 1000 times the reciprocal of the emittertemperature, i.e., cathode in degrees Kelvin, the ordinate of the graphis in terms of the current density in amperes per centimeters squares orthe current ux as it is sometimes named. The diagonal lines designatedby the Greek letter qb are ideal plots of work function, that is isowork function curves which assume materials which would retain the samework function as the temperature of the material increases. Tungsten andother refractory metals conform closely -with these curves. The solidline curve is a plot of characteristics of the present invention duringoperation. Actually, a family of such curves `would result, one for eachvapor pressure, however, for clarity only one of such curves is shown.The dotted line indicates the characteristics of a typical prior artlamp under the same conditions. It is clear that there is a substantialimprovement in performance utilizing the present invention. It isbelieved that the curves of FIGURE 4 can be explained as follows.Referring specifically to the solid line curve which representsperformance of the present invention, at the extreme right of theplotted curve electron emission is somewhat low. It is believed thatunder the conditions represented that the cathode of the device iscompletely cesiumated, that is, that a thick layer of cesium coats thecathode and that all the electrons emitted for purposes of thearc-discharge are emitted from the cesium layer and that under theseconditions the underlying base material or cathode material is notsignificant. Emission characteristics, of course, are excellent sincecesium has such a low work function. However, under these conditionsoutput is not great since the operating temperature of the electrode islow. As the temperature is increased more electrons are emitted from thecesium layer and emission increases until a maxim is reached at point A.It is believed that the conditions at point A represent a conditionlwhere the emission of free electrons for the discharge arc are stillcoming almost entirely from the cesium layer but because of the increasein temperature substantial evaporation of cesium from the cathode hasoccurred, however, a monoatomic layer of cesium remains on the cathode.Accordingly, if the temperature is increased further more cesiumevaporates from the cathode leaving less than a monoatomic layer ofcesium thereon and so the current density drops although the temperatureof the cathode increases. This drop continues over a substantialdifferential of temperature as shown by the graph until finally currentdensity starts rising rapidly and asymptotically to the iso workfunction curve for the cathode meal which in the preferred embodiment ofthe invention is tungsten having a work function of between 4.5 and 5.0.Of course, at this high out-put point the temperature is so high thatthe cathode of tungsten would rapidly deterioriate and be impracticalfor a lamp. It is significant to note, however, that prior art lamps toachieve the high outputs necessary for modern applications do indeedoperate at the high temperatures which rapidly degrade cathodeperformance. Since, as stated hereinabove, operating temperatures toachieve such outputs in prior art lamps are so high that electrodedeterioration is rapid, prior efforts have sought methods of loweringthe lwork function of the cathode material. A well-known example is theadding of an impurity to the cathode material such as in a thoriatedcathode having an oxide impurity added to the cathode material. Thedotted line of the graph indicates the curve obtained from the operationof such a typical prior art lamp which includes a cathode material suchas tungsten which has impurities added thereto. It is believed that thedegradation of performance by use of typical prior art lamps isexplained as follows. It is believed that the work function of a cathode-coated by cesium is reduced due to the dipole atomic forces existingbetween the adsorbed cesium atoms on the metal surface of the cathodeand the surface atoms themselves. When impurities such as oxides areadded to the cathode material such as in thoriated tungsten cathode, itis believed that there is a disturbance or distortion of the crystalstructure of the cathode material such that dipole forces are not aseffective in lowering the work function of the cesiumated cathode.Furthermore, as is well known cesium is a highly active material 'whichcombines readily with many other substances. Therefore, it is believedthat cesium Iwill react with the oxide normally added to lower workfunction in the typical prior art lamp, thus, effectively reducing theadvantages thought to be obtained by adding such impurities as in athoriated cathode. In fact, it is believed that as the .cesium combineswith the oxides, the work function of the cathode is raised to a pointwhich is higher than a totally untreated cathode base material.

`The present invention allows operation at a Substantially lowertemperature as in prior art devices and yet achieves a high radiatedoutput.

In a plasma arc, the high cathode temperatures found in'prior artdevices are obtained by the formation of a high voltage sheath at themetal surface of the cathode which accelerates ions generated in theplasma on to the metal surface creating the required temperature.Usually this sheath takes the form of a glowing ball at the tip of thecath-ode. The glowing ball often wanders about the cathode surfacecreating an unstable situation particularly Where the lamp is used forimaging purposes such as in photographic or calibration applications.The lamp of the present invention since it operates at substantiallylower temperatures, because of the pure refractory metal in the cathode,than prior art devices does not have this problem and, therefore, can beused for accurate imaging and calibration purposes.

There has thus been described a new and improved vaporizable materialdischarge-arc device. It is to be expressly understood that variousmodifications can be made without departing from the spirit of theinvention.

I claim:

1. A discharge-arc device comprising:

a sealed housing having first and second portions, said portions beingin communication;

a cathode and an anode disposed within said first portion of saidhousing and spaced apart to form an arc gap,

said cathode and said anode being adapted for connection to a powersource for providing a potential drop across said arc gap;

said second portion having heat choke means as an integral part thereoffor maintaining the temperature of the end of said second portion remotefrom said first portion lower during operation than the terlperature ofsaid first portion of said arc device; an

a predetermined quantity of vaporizable material within said arc device,whereby when said arc device is energized the arc is created at aparticular vapor pressure with excess material being condensed in saidsecond portion of said housing, said pressure being adjustable bycontrolling the temperature of said second portion of said housing. v

2. The discharge-arc device of claim 1 wherein said cathode isconstructed of substantially pure refractory material.

3. A discharge-arc device as claimed in claim 2, wherein said refractorymaterial is selected from the group of molybdenum, tungsten, tantalum,rhenium, zirconium and niobium and mixtures thereof.

4. A discharge-arc device as claimed in claim 3, wherein saidvaporizable material is an alkali metal.

5. A discharge-arc device as claimed in claim 4, wherein said alkalimetal is cesium.

6. A discharge-arc device as claimed in claim 1, wherein said rstportion of said housing is tubular and said electrodes are disposedcoaxially within said first portion.

7. The discharge-arc device of claim 1 wherein said second portioncomprises a metallic member having an axial bore therein for connectingsaid rst portion of said housing about said cathode and said anode witha zone in said second portion which is temperature adjustable, said heatchoke means comprising a necked down portion of said metallic memberdisposed between said first portion of said housing and said temperatureadjustable zone.

8. The discharge-arc device of claim 7 further including means disposedabout said zone in said second portion of said housing for adjustablycontrolling the temperature thereof 9. A discharge-arc devicecomprising;

a sealed housing having rst and second portions, said portions being incommunication;

a cathode and an anode disposed within said rst portion of said housingspaced apart to form an arc gap, said cathode and said anode beingadapted for connection to a power source for providing a potential dropacross said arc gap;

said second portion having means as an integral part thereof formaintaining a temperature gradient between said rst portion of saidhousing and the end of said second portion remote from said firstportion whereby the temperature of said remote end of second portion isindependent of the temperature of said rst portion; and,

a predetermined quantity of vaporizable material within said arc device,whereby when said arc device is energized an arc is created at aparticular vapor pressure with excess material being condensed in saidsecond portion of said housing adjacent said remote end, said pressurebeing adjustable by controlling the temperature of said second portionof said housing.

10. The discharge-arc device of claim 9 wherein said second portioncomprises an elongated member having an axial bore therein forconnecting said iirst portion of said housing with a zone in said secondportion which is temperature adjustable, temperature gradientmaintaining means comprising a necked down portion of said elongatedmember disposed between said rst portion of said housing and saidtemperature adjustable zone.

11. The discharge-arc device of claim 10 wherein said elongated memberis metallic.

References Cited UNITED STATES PATENTS 2,103,039 12/1937 Pirani et al.315-108 2,965,790 12/1960 Ittig et al. 313-217 2,971,110 2/1961 Schmidt313-227 JAMES W. LAWRENCE, Primary Examiner R. F. HOSSFELD, AssistantExaminer U.S. Cl. X.R.

